The invention relates to the use of non-sedating barbiturate compounds given in a manner and dosage effective to produce blood levels and brain levels of these drugs and/or their active metabolites sufficient to provide a neuroprotectant effect. In particular, the methods and formulations of the invention permit treatment of cerebral ischemia, head trauma and other acute neurologic injuries, and prevention of resulting neuronal damage.
Ischemia (stroke) is the third leading cause of death in the United States. When blood supply to the brain is reduced below a critical threshold, a cascade of biochemical events leads to irreversible damage to neurons and brain infarction. Research on treatment and prevention of ischemia is extensive but unfortunately it remains at a basic stage and no adequate therapies are yet in practice (10).
Barbiturates in high concentrations have been shown to be neuroprotective in cerebral ischemia in rodents and primates, to reduce the extent of ischemia brain infarction, and to prevent or lessen brain damage (1-4). One theory as to how barbiturates prevent neuronal injury in ischemia is that they inhibit the ischemia-induced uncontrolled release of neurotransmitters, which can attain high, neurotoxic concentrations that cause neuronal death (5).
The literature regarding the neuroprotective effects of anesthetic barbiturates is over two decades old, but the clinical use of barbiturates has been severely limited because of toxicity. The dosages and blood and brain levels necessary to confer neuroprotection are toxic and cause lethargy, stupor, and coma. Even higher doses that might be more effective are lethal (1-4, 6), making barbiturates unsuitable for treatment of ischemia (1). These toxic side effects establish a xe2x80x9cfunctional ceilingxe2x80x9d on dosage for barbiturates, and have discouraged further research into the use of anesthetic/sedative barbiturates to protect from ischemia.
Levitt et al., U.S. Pat. No. 4,628,056 describes non-sedating oxopyrimidine derivatives and their use as anticonvulsants, anti-anxiety and muscle relaxant agents. The literature does not suggest the use of such compounds as neuroprotectant agents. Indeed, even. in published studies about using sedative barbiturates for neuroprotection there is no reference to non-sedating barbiturate compounds. It is generally believed that the anticonvulsant and neuroprotective effects of barbiturates are linked to their sedative/hypnotic effects For example, Lightfoote et al. suggested that the protective effects of pentobarbital are due to the duration of the barbiturate-induced anesthesia (3). This viewpoint has been reinforced by biochemical studies at the cell receptor level that relate all these effects to action at the GABA receptor. Thus, the prior art teaches away from using sedative barbiturates for neuroprotection because of their toxicity, and also teaches away from using non-sedative barbiturates as neuroprotectants because they lack sedating or anesthetic properties.
In summary, the invention involves non-sedating barbiturates such as for example 1,3-dimethoxymethyl 5,5-diphenyl-barbituric acid (DMMDPB), 1-monomethoxymethyl 5,5-diphenylbarbituric acid (MMMDPB) an diphenyl-barbituric acid (DPB) and their precursors, derivatives and analogs, and their administration over a range of dosages that result in a range of blood levels and brain levels of these drugs and their metabolites making them useful as neuroprotectants. In particular, the invention is directed to the treatment of cerebral ischemia, head trauma and other acute neurologic injuries, using non-sedating barbiturates.
There are many circumstances where individuals at risk of cerebral ischemia are clearly identified in advance, for example: individuals undergoing cardiac surgery or carotid endarterectomy, and individuals with atrial fibrillation, transient ischemic attacks (TIAs), bacterial endocarditis, strokes, or subarachnoid hemorrhage due to a cerebral aneurysm. In such cases, a non-sedating barbiturate is used prophylactically in individuals at risk for ischemic damage. The drugs can also be used after an acute event. These compounds can be given in oral form as a tablet, capsule, liquid or via intravenous or other parental routes.
This invention succeeds where previous efforts to treat cerebral ischemic attack with barbiturates have failed. This invention solves a problem previously thought to be insoluble, that of toxic effects of neuroprotective dosages of barbiturates. The invention avoids the toxicity and sedative effects of barbiturates known in the prior art without loss of efficiency.
This invention satisfies a long-felt need for a non-toxic neuroprotectant, and is contrary to the teachings of the prior art regarding the inability of barbiturates to produce clinically meaningful neuroprotection. According to the invention, it is possible to separate the anticonvulsant and sedative effects of barbiturates, and neuroprotection correlates much better with the anticonvulsant rather than the sedative effect of barbiturates.
This invention differs from the prior art in the recognition of specific compounds, their modifications and dosages that are effective in neuroprotection but that were not previously recognized.
The present invention is a method for providing neuroprotection to a mammal, preferably a human. The method comprises administering to the mammal a non-sedating barbiturate in a dose effective to provide a neuroprotection effect. Non-sedating barbiturates for use in the invention include one or more selected from the group consisting of 1,3-dimethoxymethyl 5,5-diphenyl-barbituric acid (DMMDPB), 1-monomethoxymethyl 5,5-diphenylbarbituric acid (MMMDPB), and diphenyl barbituric acid. The precursor, derivatives and analogs of the foregoing compounds, as well as the salts of all the foregoing are also suitable for practicing the invention.
The effective neuroprotective dose of the non-sedative barbiturate preferably exceeds the coma-producing dose of a sedative barbiturate. Depending on the specific need of the mammal, the dose of the non-sedative barbiturate may exceed a dose that would be lethal with a sedative barbiturate. This unexpected and seemingly paradoxical effect of the present method is further reflected in the relative dosage levels that are possible with the methods of this invention.
Also, the neuroprotective dose of the non-sedative barbiturate exceeds the minimum anticonvulsant dosage of the barbiturate. In some embodiments of the present invention the effective dose of the non-sedative barbiturate is in the range of from about 2 times to about 5 times the anticonvulsant dosage. In yet other contexts where the need of the mammal requires, the effective dose of the non-sedative barbiturate is in the range of from about 5 times to about 10 times the anticonvulsant dosage of the non-sedative, or even higher so long as the dose is clinically acceptable.
Advantageously, the neuroprotective effect of the present methods can be used to mitigate the effect of cerebral ischemic. The non-sedating barbiturate can be administered orally, intravenously, transdermally, in combination with an adjuvant, or transpulmonarily by means of a particulate or aerosol inhalant. Moreover, within the scope of the invention, the non-sedating barbiturate can be administered preventively, prophylactically or therapeutically, at a clinically acceptable dose. The compound may be administered prophylactically before evident neuronal damage, or therapeutically after onset of neuronal damage. The neuroprotective effect diminishes, or protects the subject from, neuronal damage caused by head trauma or cerebral ischemia. The compound may be administered in conjunction with cardiac surgery or carotid endarterectomy. The mammalian subject may have or be at risk for atrial fibrillation, a transient ischemic attack (TIA), bacterial endocarditis, a stroke, head trauma, or subarachnoid hemorrhage.
Typically, to achieve neuroprotection the non-sedating barbiturate is administered in a dose sufficient to obtain blood concentrations of at least about 30 xcexcg/ml of barbiturate, preferably at least about 100 xcexcg/ml, more preferably at least about 250 xcexcg/ml, and possibly as high as 200-300 xcexcg/ml, or even higher. In contrast, the reported therapeutic range for phenobarbital is lower, 10-30 xcexcg/ml blood levels (6). Thus, preferred ranges are at or above about 25, 30, 50, 75, 100, 200, 250, or 300 xcexcg/ml.
The invention includes a pharmaceutical composition comprising a non-sedating barbiturate administered in an amount effective to have a neuroprotectant effect. Preferably, the non-sedating barbiturate is administered in oral doses in the range of from about 25 to about 1,500 mg/kg/day body weight. Preferably the dose is greater than about 25 mg/kg/day, or greater than about 100 mg/kg/day, or greater than 250 mg/kg/day. A preferred dose is one that is pharmacologically equivalent to a dose of about 1000 mg/kg/day in the rat. Thus, dosage forms may be sufficient individually or in multiple doses to provide a dose equal to or above about 15, 20, 25, 50, 70, 100, 250, 500, 1000, or 1500 mg/kg body weight per day.
In human trials it has been unexpectedly found that DMMDPB, one of the neuroprotectant compounds, is much better absorbed in humans than in rats or dogs. It has further been found that the half life of DMMDPB, as well as the half life of MMMDPB and DPB are greater than the half-lives found in rats or dogs. Specifically, with dosages of 20 mg/kg/day, the half-lives of DMMDPB, MMMDPB, and DPB are approximately 20 hrs, 20 hrs, and 50 hrs respectively after a two week exposure in humans. Similarly, the maximum concentration (Cmax) of the drug in the blood following 7 days of dosing in the range of 20 mg/kg/day are 1.2 xcexcg/ml, 36 xcexcg/ml and 43 xcexcg/ml respectively.
The unexpectedly high absorption and prolonged half-life in humans makes it possible to achieve substantial blood levels with lower than expected oral dosages. Thus, for example, it is possible to obtain total barbiturate blood levels (i.e., DMMDPB+MMMDPB+DPB) greater than 53 xcexcg/ml with dosages of about 15 mg/kg/day; and total barbiturate levels greater than 72 xcexcg/ml with dosages in the range of 20 mg/kg/day. Blood levels of non-sedating barbiturates greater than 100 xcexcg/ml are achieved with dosages between about 40 and about 100 mg/kg/day, and are within the scope of the invention. With parenteral administration of non-sedating barbiturates, similar blood concentrations are obtained with daily dosages of less than 25 mg/kg/day. However, first day loading dosages may still need initial dosages of greater than 25 mg/kg.
The invention provides an article of manufacture comprising a container comprising a pharmaceutical composition and a label with indications for use as a neuroprotectant, the pharmaceutical composition comprising a non-sedating barbiturate compound in an amount effective for neuroprotection upon administration to a subject in need of neuroprotection; and a pharmaceutically acceptable carrier or excipient.
Another embodiment is a method for providing neuroprotection comprising (a) diagnosing a patient""s need for cerebral neuroprotection, (b) selecting a non-sedative barbiturate, and (c) providing to the patient a dose of the non-sedative barbiturate sufficient to raise the concentration in the patient""s brain to a level effective to provide neuroprotection.
Further objectives and advantages will become apparent from a consideration of the description and examples.
In describing preferred embodiments of the present invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. It is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. Each reference cited here is incorporated by reference as if each were individually incorporated by reference.
The term xe2x80x9cnon-sedative barbituratexe2x80x9d encompasses the family of 5,5-diphenyl barbituric acid anticonvulsant compounds described in Levitt et al., U.S. Pat. No. 4,628,056, and metabolic precursors and metabolites, and derivatives and structural analogs (including addition salts thereof) having a non-sedative neuroprotectant activity. Other barbituric acid derivatives that are non-sedating are also within the scope of the invention.
Derivatives, precursors, and analogs of barbituric acid include barbituric acids of the formula: 
wherein one or more nitrogen is substituted with lower alkyl, or a lower alkoxy substituted lower alkyl group; or at least one of R1 and R2, together with the nitrogen, forms a carbamate, an amide, or an acetal of the formamide derivative, i.e. R1 or R2 is CO2R, COR or CH(OR)2. Methylether groups are preferred R1 and R2 groups and methoxymethyl is more preferred. Methyl is also a preferred value for R1 and/or R2. Other derivatives of barbituric acids according to the invention are carbamates, amides and acetals where one or both of R1 and R2 is CH2OR5, wherein R5 is lower alkyl, alkylaryl or benzyl; CO2R6, wherein R6 is lower alkyl or aryl; COR7, wherein R7 is hydrogen, lower alkyl or aryl; or CH(OR8)2, wherein R8 is a lower alkyl group.
Preferred values for R3 and R4 are aryl, phenyl, phenyl substituted with a halogen or lower alkyl group, benzyl, benzyl wherein the aromatic ring is substituted with a halogen or lower alkyl group, lower alkyl or lower alkyl substituted with an aromatic moiety. Aryl represents any carbocyclic ring, such as phenyl, naphthyl and higher analogues, as well as heteroaromatic rings substituted with one or more heteroatoms such as sulfur, oxygen and nitrogen. According to the invention, nonsedating barbituric acid derivatives are those where at least one of R3 and R4 is an aromatic ring or an aromatic ring containing moiety e.g. aryl, phenyl, substituted phenyl, benzyl, substituted benzyl or arylalkyl. Preferred substituents on the aromatic rings are methyl, ethyl, and fluorine. Phenyl and substituted phenyl are preferred for R3 and R4. Embodiments where R3 and R4 are both phenyl are most preferred.
In preferred compounds, one of R1 and R2 is hydrogen, or one or both of R1 and R2 is methyl or alkoxymethyl, preferably methoxymethyl. At least one and preferably both of R3 and R4 is preferably phenyl or substituted phenyl, tolyl, fluorophenyl, ethylphenyl.
As can be readily understood, salts of the above compounds are also contemplated, including organic salts, such as acid addition and base addition salts.
In order to fall within the scope of this genus, the compound must (1) be a barbituric acid chemical derivative, (2) not be sedating, in the sense that the subject remains awake and alert at useful doses, that is, not anesthetized, and (3) manifest neuroprotective activity in an animal model described herein or in a human at a dose that is not toxic to the relevant animal species, or show activity in an in vitro assay now known or later discovered that is accepted as a model for in vivo neuroprotection.
These barbituric acid derivatives may be both prodrugs and active ingredients in the subject, thus combining to produce the desired pharmacodynamic effect of neuroprotection. Sustained levels are readily obtained with such compounds.
Thus certain barbiturate compounds have been developed and have anticonvulsant activity without being sedating even at very high brain concentrations (that would be lethal with other barbiturates). According to the invention, such compounds are used to neuroprotect an animal at risk for or suffering from one or more ischemic episodes such as that modeled by middle cerebral artery occlusion, while these compounds do not cause the toxic effects of other barbiturates when present at concentrations required for prevention of ischemic brain damage.
As described herein, non-sedative barbiturate drugs lessen or prevent ischemic brain damage in a rat model of focal cerebral ischemia produced by middle cerebral artery occlusion. This demonstrates utility in humans.
In a reproducible, predictive model of cerebral ischemia known in the art, selective neuronal damage is produced in the striatum and cerebral cortex by bilateral carotid occlusion accompanied by systemic hypotension. The resulting cerebral ischemia causes a release of excitotoxic neurotransmitters and dopamine in striatum. Pentobarbital inhibited this ischemia-induced release, pointing to one possible mechanism of barbiturate neuroprotection. (5) A neuroprotective dose of pentobarbital was found to be 70 mg/kg. Inhibition of neurotransmitter release by several neuroprotective anesthetic agents (isoflurane, etomidate, propofol) was also known.
The above and similar animal models (see Examples) can be used
(1) to analyze whether a non-sedative barbiturate with anticonvulsant properties but little or no anesthetic activity can provide neuroprotection in the striatum or hippocampus, and
(2) to determine if the agent prevents or reduces release of neurotransmitters in response to ischemia. Uncontrolled or unmodulated neurotransmitter release is one of the postulated mechanism of ischemic damage. For non-sedating barbiturates that inhibit release of neurotransmitters, this approach can serve as a biochemical assay for predicting utility of a compound according to the invention, and the invention encompasses such methods.
A neuroprotective effect according to the invention can be demonstrated and characterized by performing a dose-response study and measuring statistically significant differences in neuronal damage at the various doses of the drug. Dose-response curves generated in such studies can be used to compare the relative degree of neuroprotection and sedation of a test compound.
Cerebral ischemia is induced in rats by occlusion of the middle cerebral artery (xe2x80x9cMCAxe2x80x9d) (7-9). The occlusion can be performed in an irreversible or reversible manner. In the latter case, after a period of obstruction, blood flow is restored. These animal preparations are thus appropriate to model various types of strokes in humans and to permit determination of a drug""s neuroprotective action. Such models permit observation of the prevention of brain damage and the evaluation of the drugs as being useful for humans who are at risk for ischemic stroke for reduction of subsequent brain damage induced by an ischemia event. Because they prevent brain damage in models of irreversible ischemia and reversible ischemia with restoration of blood flow, the compounds of the invention are also useful for treating acute ischemic stroke either alone or in combination with other agents, for example, thrombolysins such as tissue plasminogen activator that reduce the extent of brain infarction when circulation is restored.
The term xe2x80x9ctreatmentxe2x80x9d is intended to encompass administration of compounds according to the invention prophylactically to prevent or suppress an undesired condition, and therapeutically to eliminate or reduce the extent or symptoms of the condition. Treatment according to the invention is given to a human or other mammal having a disease or condition creating a need of such treatment. Treatment also includes application of the compound to cells or organs in vitro. Treatment may be by systemic or local administration.
The non-sedative barbiturate compositions of the present invention, may be formulated into xe2x80x9cpharmaceutical compositionsxe2x80x9d with appropriate pharmaceutically acceptable carriers, excipients or diluents. If appropriate, pharmaceutical compositions may be formulated into preparations including, but not limited to, solid, semi-solid, liquid, or gaseous forms, such as tablets, capsules, powders, granules, ointments, solutions, suppositories, injections, inhalants, and aerosols, in the usual ways for their respective route of administration.
Methods known in the art can be used to achieve time-release of the composition or to prevent metabolism, release or absorption of the composition until it has reached its intended target site. A pharmaceutically-acceptable formulation should be employed that does not inactivate the active drug of the present invention.
In pharmaceutical dosage forms, the compositions may be used alone or in appropriate association, as well as in combination with, other pharmaceutically-active compounds.
The pharmaceutical compositions of the present invention can be delivered via various routes and to various sites in an animal body to achieve the desired neuroprotective effect. Local or systemic delivery can be accomplished by injection, infusion, application or instillation of the composition into one or more body cavities, or by inhalation or insufflation of an aerosol. Parenteral administration can be by intramuscular, intravenous, intraperitoneal, subcutaneous intradermal, or topical administration.
The compositions of the present invention can be provided in unit dosage form, wherein each dosage unit, e.g., a teaspoon, tablet, solution, or suppository, contains a predetermined amount of the active drug or prodrug, alone or in appropriate combination with other pharmaceutically-active agents. The term xe2x80x9cunit dosage formxe2x80x9d refers to physically discrete units suitable as unitary dosages for human and animal subjects, each unit containing a predetermined quantity of the composition of the present invention, alone or in combination with other active agents, calculated in an amount sufficient to produce the desired effect, in association with a pharmaceutically-acceptable diluent, carrier (e.g., liquid carrier such as a saline solution, a buffer solution, or other physiological aqueous solution), or vehicle, where appropriate. The specifications for the novel unit dosage forms of the present invention depend on the particular effect to be achieved and the drug""s particular pharmacodynamics in the particular host.
An xe2x80x9ceffective amountxe2x80x9d of the composition is that required to produce the desired pharmacologic effect in a host. This can be monitored using any of a number of end-points known to those skilled in the art. The xe2x80x9ceffective dosexe2x80x9d will depend on the bioavailability of specific dosage forms delivered by one or another route of administration. The neuroprotective dosage and blood level of the present compounds is at least 2-fold and preferably at least about 5 to 10-fold the anticonvulsant dosage of a sedating barbiturate. Based on rat data, the anticonvulsant ED50 for phenobarbitol is about 50-100 mg/kg. A non-sedating barbiturate dose of 1 g/kg given over 7 days protects against cerebral ischemia in the rat. Similar or lower doses are suitable in humans based on the enhanced absorption in humans discussed above.
The amount of each active agent employed in the Examples below provides general guidance for the range that can be utilized by the skilled practitioner to optimize the doses and methods of the present invention. Moreover, such dose ranges do not preclude use of a higher or lower dose of a component, as might be warranted in a particular application. For example, the actual dose and schedule may vary depending on whether the compositions are administered in combination with other drugs, or depending on inter-individual differences in pharmacokinetics, drug disposition, and metabolism. Similarly, amounts may vary for in vitro applications. It is within the skill in the art to readily adjust the dose in accordance with the necessities of a particular situation without undue experimentation.
Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting of the present invention, unless specified.